Audio amplifier with timing circuit to power down bias control and amplifying circuits in sequence

ABSTRACT

An audio amplifier includes a timing control circuit, an amplifying circuit, and a bias control circuit. The timing control circuit generates a first power down signal and a second power down signal, in which the first power down signal is asserted before the second power down signal is asserted. The amplifying circuit receives a bias voltage to amplify an audio signal and is deactivated when the first power down signal is asserted. The bias control circuit provides the bias voltage for the amplifying circuit and is deactivated when the second power down signal is asserted.

BACKGROUND

1. Field of Invention

The present invention relates to a power amplifier. More particularly,the present invention relates to a power amplifier for amplifying anaudio signal.

2. Description of Related Art

As the name suggests, an audio amplifier amplifies audio signals. Theseaudio amplifiers typically include an audio input stage and an audiooutput stage. The audio input stage is connected to some type of audiosource and the audio output stage is connected to the audio device suchas a speaker. These audio amplifiers receive audio signals from theaudio source, amplify those audio signals, generate audio currentsignals based on those amplified signals, and output the audio currentsignals to the speaker. The audio current signals drive the speaker andcause the speaker to reproduce the audio signals that are generated bythe audio source to create sound.

To amplify the audio signal, the amplifying circuit of the audioamplifier typically requires a bias circuit to provide bias voltages tobias the amplifying circuit. However, when the audio amplifier justpowers on, the power source, such as the supply voltage, is suddenlyapplied to the bias circuit. This causes the bias voltage generated bythe bias circuit to vibrate a lot. As a result, the output audio signalgenerated according to the bias voltage vibrates as well. This mightdamage the audio amplifier and produce unpleasant pop noises.

Therefore, there is a need for a new audio amplifier which can preventthe pop noise, and prevent the amplifier circuit from being damaged whenthe audio power amplifier powers on or powers down.

SUMMARY

According to one embodiment of the present invention, an audio amplifieris disclosed. The audio amplifier includes a timing control circuit, anamplifying circuit, and a bias control circuit. The timing controlcircuit generates a first power down signal and a second power downsignal, in which the first power down signal is asserted before thesecond power down signal is asserted. The amplifying circuit receives abias voltage to amplify an audio signal and is deactivated when thefirst power down signal is asserted. The bias control circuit providesthe bias voltage to the amplifying circuit and is deactivated when thesecond power down signal is asserted.

According to another embodiment, a method for reducing an explosionnoise generated from an audio amplifier is disclosed. The method firstpowers off the amplifying circuit of the audio amplifier, in which theamplifying circuit amplifies the audio signal and then the audioamplifier bias control circuit is powered off after the amplifyingcircuit has been powered off, in which the bias control circuit providesat least one bias voltage to the amplifying circuit.

It is to be understood that both the foregoing general description andthe following detailed description are by examples, and are intended toprovide further explanation of the invention as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features, aspects, and advantages of the presentinvention will become better understood with regard to the followingdescription, appended claims, and accompanying drawings where:

FIG. 1 shows the block diagram of the audio amplifier according to oneembodiment of the present invention;

FIG. 2 shows the block diagram of the audio amplifier according toanother embodiment of the present invention;

FIG. 3 shows the flow chart of method for playing an audio signalaccording to the embodiment of the present invention; and

FIG. 4 shows the waveforms of the audio amplifier according to theembodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Reference will now be made in detail to the present preferredembodiments of the invention, examples of which are illustrated in theaccompanying drawings. Wherever possible, the same reference numbers areused in the drawings and the description to refer to the same or likeparts.

The timing control circuit of the audio amplifier in the followingembodiment turns off the bias control circuit only when the amplifyingcircuit has been turned off previously, such that the bias voltageprovided to the amplifying circuit doesn't vibrate when the audioamplifier is powered off. Therefore, the pop noise of the output audiosignal can be eliminated.

FIG. 1 shows the block diagram of the audio amplifier according to oneembodiment of the present invention. The audio amplifier includes atiming control circuit 103, an amplifying circuit 105, and a biascontrol circuit 101. The timing control circuit 103 generates a firstpower down signal PWD1 and a second power down signal PWD2, in which thefirst power down signal PWD1 is asserted before the second power downsignal PWD2 is asserted. The amplifying circuit 105 receives a biasvoltage VB, amplifies the audio signal VI, and is deactivated when thefirst power down signal PWD1 signal is asserted. The bias controlcircuit 101 provides the bias voltage VB for the amplifying circuit 105and is deactivated when the second power down signal PWD2 is asserted.

FIG. 2 shows the block diagram of the audio amplifier according toanother embodiment of the present invention. The audio amplifierincludes the timing control circuit 203, the amplifying circuit 205, andthe bias control circuit 201. The amplifying circuit 205 receives a biasvoltage VB to amplify the audio signal VI, and is deactivated when thefirst power down signal PWD1 is asserted. The common input voltageV_(CM) inputted to the amplifying circuit 205 can be either the supplyvoltage or the ground voltage. The bias control circuit 201 provides thebias voltage VB for the amplifying circuit 205, and is deactivated whenthe second power down signal PWD2 is asserted.

The timing control circuit 203 generates the first power down signalPWD1 and the second power down signal PWD2. In this embodiment, thefirst power down signal PWD1 is generated by delaying the original powerdown signal PWD with the inverter 213, and the second power down signalPWD2 is generated by delaying the first power down signal PWD 1 with theinverter 215. Therefore, the first power down signal PWD1 is assertedbefore the second power down signal PWD2 is asserted.

The amplifying circuit 205, amplifying the audio signal VI, includes afirst operation amplifier 207 and a second operation amplifier 209. Thefirst operation amplifier 207 has a first bias end for receiving thebias voltage VB. The first bias end is tied to a supply voltage or aground voltage when the timing control circuit powers off the amplifyingcircuit. The first operation amplifier 207 generates the invertingoutput voltage Von according to the audio signal VI and the bias voltageVB. The first operation amplifier 207 generates the inverting outputvoltage Von only when the first power down signal PWD1 is de-assertedand the bias control circuit 201 provides proper bias voltage VB.

The second operation amplifier 209 has a second bias end for receivingthe bias voltage VB. The second bias end is tied to a supply voltage ora ground voltage when the timing control circuit powers off theamplifying circuit. The second operation amplifier 209 generates thenon-inverting output voltage Vop according to the inverting outputvoltage Von and the bias voltage VB. The second operation amplifier 209generates the inverting output voltage Vop only when the first powerdown signal PWD1 is de-asserted and the bias control circuit 201provides proper bias voltage VB. The output voltage Vop and the outputvoltage Von drive the speaker 211 to reproduce the audio signal VI.

When the audio amplifier is just powered down, the bias control circuit201 is powered down only after the amplifying circuit 205 has beenpowered down. Because the bias control circuit 201 still provides theproper bias voltage to the amplifying circuit 205 when the amplifyingcircuit 205 begins powering down, so the amplifying circuit 205 canstill generate the inverting output voltage Von and the non-invertingoutput voltage Vop normally, and the pop noises of the output audiosignal can be eliminated.

The first operation amplifier 207 and the second operation amplifier 209can be class AB amplifiers, class B amplifiers, or class A amplifiers.The power transistors on the class A amplifier are in conductive statesall the time, which means that the power transistors of the class Aamplifier continuously dissipate power. Thereby, the class A amplifierhas low power efficiency.

The power transistors on the class B amplifier are successively drivenbetween conductive and non-conductive states, therefore, the powertransistors of the class B amplifier operate only 50% of the time. Thepower transistors on the class AB amplifier are in the conductive statesfor time periods greater than one-half of the total period. Thereby, thepower transistors of the Class AB amplifier are operated somewherebetween 50% of the time and all of the time. As a result, because ClassAB amplifier and Class B amplifier have better power efficiency, theyare used more often in the audio amplifier than class A amplifier.

The amplifying circuit 205 further includes the first resistor R1, thesecond resistor R2, the third resistor R3, and the fourth resistor R4.The first resistor R1 has one end receiving the audio signal VI, and theother end connected to a negative terminal of the first operationamplifier 207. The second resistor R2 connected between the negativeterminal and the output terminal of the first operation amplifier 207.The third resistor R3 has one end connected to the output terminal ofthe first operation amplifier 207 and the other end connected to anegative terminal of the second operation amplifier 209. The fourthresistor R4 connected between the negative terminal and the outputterminal of the second operation amplifier 209. Tuning the resistance ofthe resistor R1\R2\R3\R4 changes the value of the voltage gain Von/VIand the voltage gain Vop/VI.

FIG. 3 shows the flow chart of method for playing an audio signalaccording to the embodiment of the present invention. The method reducesthe explosion noise generated by the audio amplifier. The method assertsthe first power down signal (step 301), and powers off the amplifyingcircuit of the audio amplifier according to the asserted first powerdown signal (step 303). The amplifying circuit can be powered off bypassing a ground voltage or a supply voltage to the bias end of theamplifying circuit.

Then, the method asserts the second power down signal after theamplifying circuit has been powered off (step 305), and powers off thebias control circuit of the audio amplifier according to the assertedsecond power down signal (step 307), in which the bias control circuitprovides at least one bias voltage for the amplifying circuit. Delayingthe first power down signal with an inverter generates the second powerdown signal.

FIG. 4 shows the waveforms of the audio amplifier according to theembodiment of the present invention. According to the waveform, we cansee that if the first power down signal PWD1 powers off the amplifyingcircuit before the second power down signal PWD2 powers off the biascontrol circuit, the explosion noises of the output audio signal isreduced at time t1 when the audio amplifier is powered down.

According to the above embodiments, the timing control circuit of theaudio amplifier turns off the bias control circuit only when theamplifying circuit has been powered off previously, such that the biasvoltage provided to the amplifying circuit doesn't vibrate when theaudio amplifier is powered off. Therefore, the pop noise of the outputaudio signal can be eliminated.

It will be apparent to those skilled in the art that variousmodifications and variations can be made to the structure of the presentinvention without departing from the scope or spirit of the invention.In view of the foregoing, it is intended that the present inventioncover modifications and variations of this invention provided they fallwithin the scope of the following claims and their equivalents.

1. An audio amplifier, comprising: a timing control circuit generating afirst power down signal and a second power down signal; an amplifyingcircuit coupling with the timing control circuit, wherein the amplifyingcircuit receives a bias voltage to amplify an audio signal and receivesthe first power down signal; and a bias control circuit coupling withthe timing control circuit and the amplifying circuit, wherein the biascontrol provides the bias voltage for the amplifying circuit andreceives the second power down signal, wherein the first power downsignal is asserted before the second power down signal is asserted, andthe amplifying circuit is deactivated when the first power down signalis asserted, and the bias control circuit is deactivated when the secondpower down signal is asserted.
 2. The audio amplifier as claimed inclaim 1, wherein the timing control circuit comprises: a first invertergenerating the first power down signal by delaying an original powerdown signal; and a second inverter generating the second power downsignal by delaying the first power down signal.
 3. The audio amplifieras claimed in claim 1, wherein the amplifying circuit comprises: a firstoperation amplifier having a first bias end for receiving the biasvoltage, and generating an inverting output voltage according to theaudio signal and the bias voltage; and a second operation amplifierhaving a second bias end for receiving the bias voltage, and generatinga non-inverting output voltage according to the inverting output voltageand the bias voltage, wherein the first bias end and the second bias endare tied to a supply voltage or a ground voltage when the timing controlcircuit powers off the amplifying circuit.
 4. The audio amplifier asclaimed in claim 3, wherein the amplifying circuit further comprises: afirst resistor having one end receiving the audio signal and the otherend connected to an negative terminal of the first operation amplifier;and a second resistor connected between the negative terminal and anoutput terminal of the first operation amplifier.
 5. The audio amplifieras claimed in claim 4, wherein the amplifying circuit further comprises:a third resistor having one end connected to the output terminal of thefirst operation amplifier and the other end connected to a negativeterminal of the second operation amplifier; and a fourth resistorconnected between the negative terminal and an output terminal of thesecond operation amplifier.
 6. The audio amplifier as claimed in claim3, wherein the first operation amplifier and the second operationamplifier are class AB amplifiers.
 7. The audio amplifier as claimed inclaim 3, wherein the first operation amplifier and the second operationamplifier are class B amplifiers.
 8. The audio amplifier as claimed inclaim 3, wherein the first operation amplifier and the second operationamplifier are class A amplifiers.